The present invention relates to a bias control circuit for an audio amplifier utilizing as an amplifier element a field effect transistor (hereinafter referred to as "FET"), more particularly, a vertical or unsaturated junction type FET.
In the field of an audio amplifier, there has recently been accepted the type which uses an unsaturated junction type FET as an active circuit element constituting its final or power amplifier stage. Namely, an unsaturated junction type FET is an active circuit element showing the so-called triode tube characteristic different from the prior art saturated type FET which indicates the so-called pentode tube characteristic. Therefore said unsaturated junction type FET has an excellent advantage in amplifying audio signals of a particularly large input level.
On the other hand, an unsaturated junction type FET has the disadvantage that the amount of its drain current I.sub.D varies with the drift of its drain-to-source voltage V.sub.DS. Consequently, an audio amplifier containing an unsaturated junction type FET involves the problem that the amount of its drain current I.sub.D varies with its power supply voltage, even if its gate-to-source voltage V.sub.GS is kept at a constant value, resulting in the crossover distortion of a signal amplified by the FET as well as the undue increment of its temperature.
Accordingly, practical application of an audio amplifier containing an unsaturated junction type FET is primarily governed by the extent to which the drift of its drain current I.sub.D can be reduced with respect to the fluctuation of its power supply voltage.
Thus a counter measure hitherto adopted for an audio amplifier containing an unsaturated junction type FET to prevent the drift of its drain current I.sub.D with respect to the fluctuation of its power supply voltage is exemplified by one in which power sources for the driver stage (usually constituted by a bipolar transistor) and the final power amplifier stage formed of the FET are respectively constituted by constant voltage sources. From the technical as well as the economical viewpoint, it raises little problem that a power source for the driver stage consuming a relatively small amount of power is constituted by a constant voltage source. Where, however, a power source for the power amplifier stage consuming a maximum amount of power is constituted by a constant voltage source, then some technical difficulties arise in addition to the required use of expensive circuit elements of a large current capacity as the constituent elements of the constant voltage source.
For this reason, an audio amplifier utilizing an unsaturated junction type FET is desired to have such a circuit arrangement that the amount of drain current I.sub.D flowing into the FET can always be kept substantially constant, without employing a constant voltage source for the power amplifier stage including the FET, even if the level of its power supply voltage is fluctuated by any cause.
Such a circuit arrangement may include the type which enables the level of the gate-to-source potential V.sub.GS of the FET to drift at substantially the same rate as its drain-to-source voltage V.sub.DS, even if the level of the power supply voltage is fluctuated. To provide a practical circuit arrangement for the above-mentioned object, those skilled in the art can readily think of a process of employing the power sources whose voltages always fluctuate at a substantially equal level for the driver and power amplifier stages, without utilizing constant voltage sources.
However, such a circuit arrangement can not yet maintain the drain current I.sub.D flowing into the FET at a constant value even if the levels of the power supply voltages are fluctuated. The reason is that voltage V.sub.GS applied between the gate and source of the FET varies at a higher level than that of voltage V.sub.DS impressed between the drain and source thereof due to the voltage amplification factor .mu. of the FET.